In the disclosure of the present invention reference is mostly made to the treatment of diabetes, however, this is only an exemplary use of the present invention.
The most common type of drug delivery device adapted to deliver a given drug subcutaneously is in the form of a pen-formed drug delivery device adapted to expel a desired amount of drug from a loaded cartridge by moving a cartridge piston in a distal direction to thereby expel drug via an attached subcutaneous needle. The means for moving the piston is typically in the form of a piston rod driven distally by an expelling mechanism. The device may be supplied as a “pre-filled” disposable device in which the drug-filled cartridge cannot be exchanged and the device is to be discarded when the cartridge has been emptied, or as a “durable” reusable device in which a cartridge can be inserted and subsequently removed by the user in order to allow a new cartridge to be inserted. A further distinction can be made between “manual” drug delivery devices in which the energy for driving expelling mechanism is supplied directly by the user during injection, and “automatic” drug delivery devices in which an energy source is released by the user to thereby drive the expelling mechanism. The energy source may be in the form of a spring which is strained by the user during setting of a desired dose. The spring-driven devices may be pre-filled as the FlexTouch® from Novo Nordisk or durable as the ServoPen® from Ypsomed. Alternatively, a pre-filled device may be provided with a pre-strained spring adapted to expel the full content of the cartridge, the individual doses being set by the user. The present invention addresses issues related to spring-driven drug delivery devices of the durable type, which may be either of the traditional type with a detachable cartridge holder or be front-loaded, as well as drug delivery devices of the prefilled type.
During normal operation the drug product has to flow through a narrow needle during the dispensing operation, the viscosity of the drug thereby opposing the force of the piston drive element, e.g. a piston rod, so that the drive element and the additional components of the expelling mechanism remain at a relatively low speed level during expelling of a dose of drug. The relatively low speed level results in relatively low impulses or impacts when one moving part or component hits another part or component in the injection device, which has a positive effect on the service life of the device. However, this is only true if a drug cartridge has been inserted in the device. If a user initiates a drug dispensing operation without an inserted cartridge there will be no damping effect. This can lead to very high acceleration values for the components in the expelling mechanism of the device, especially for rotating components, causing high-energy impacts when components make contact with one another, this potentially having a negative effect on the service life of the device. As a result, the device may be damaged so that it can no longer be used to administer drug or, even worse, be damaged so that an incorrect amount of drug is expelled. Increasing the dimensions accordingly, e.g. to absorb or counter impacts on or between moving parts, would typically increase the size of the device and/or result in additional costs.
The above-described situation may also arise when a new cartridge is inserted without the piston drive element contacting the piston. This will be the case if, for example, the drive element is pushed too far back during the process of changing the cartridge or if a partially full cartridge is inserted, or drug has leaked out of the cartridge through an attached needle. In these situations, too, extreme acceleration values can occur during the idle stroke of the drive element, e.g. the portion of the stroke performed by the drive element until it makes contact with the cartridge piston. Further, if a needle is not mounted on the cartridge in such a situation, the drug in the cartridge is compressed due to the impact of the drive element, this potentially causing damage, e.g. fracture, of an inserted glass cartridge. Correspondingly, a prefilled device may be supplied to a user with a gap between the piston and the drive element.
The above issue has been addressed in US 2011/054412 and US 2011/0077595 disclosing a pen-type drug delivery device with a brake mechanism in which a brake element is moved back and forth during rotational movement of the expelling mechanism, this resulting in mechanical energy being dissipated as heat and thereby a braking action on the expelling mechanism. GB 2 443 390 discloses an automatic type pen device with a manual user-actuated brake element. US 2004/0215153 and U.S. Pat. No. 5,480,387 disclose pen-type drug delivery devices with brake elements in the form of static friction elements.
Having regard to the above, it is an object of the present invention to provide a drug delivery device with a brake mechanism which is compact, effective and reliable. The brake arrangement should be cost-effective and allow a high degree of freedom of design for the device and the incorporated expelling mechanism.